There are several difficulties in implementing demodulators for such ASK coded or AM signals.
First, the signal received at a receiver input RX (see FIG. 2) has a quite large dynamic range. The carrier signal peak-to-peak amplitude can vary from rail to rail voltages down to several mV amplitude depending on the field strength and on the distance to the transmitting device, respectively.
Second, the carrier frequency of 13.56 MHz is quite close to the baseband signal frequencies (which ranges up to 848 kHz). This makes the design of filters for suppressing the carrier signal rather complex. A typical approach for demodulation is rectification by means of a peak detector using a diode. The disadvantage of this approach is that the rectification has bad efficiency for small carrier amplitudes which limits the operating range.
Yet another approach would be synchronous demodulation with a mixer and a low pass filter. This known technology typically needs rather complex and power consuming components like a clock source (crystal, PLL) with low phase noise, mixers and a filter.
WO 97/48180 discloses an envelope detector including sample-and-hold circuitry controlled by preceding carrier pulse peaks. A sampling synchronous envelope detector adopts a specialized sample-and-hold (“S&H”) approach, basing a detected output on instantaneous values of the carrier waveform which are sampled at specially chosen instants. Non-linear distortion is avoided by timing the sampling instants to occur at or near a carrier wave peak which is subsequent to an earlier carrier wave peak which serves as a time base. Sampling instants occur only at or near positive carrier peaks (or only at or near negative peaks) in a half-wave embodiment, and sampling instants occur at or near both positive and negative carrier peaks in a full wave embodiment. Another aspect of the detector provides means, such as a phase locked loop, for ensuring that the phase of the sampling instants is maintained continuously, even in the event of carrier pinch-off or other event which distorts or minimizes the carrier waveform from which the timing instants would otherwise be determined. Still another aspect of the detector provides for low pass filtering, and group delay equalization of the filtered signal, before it is output.
The known device, however, uses an approach to delay the sampling point by constant delay times to a time point close to the peak of the signal to be sampled, but not exactly to the peak of the signal. There is no provision to adapt the delay times such that sampling takes place exactly at the occurrences of the peaks of the signal to be sampled.